The IBM SELECTRIC Composer: Proportional Escapement Mechanism

The IBM SELECTRIC Composer's escapement system employs rotating elements, rather than the conventional rack system, to provide the required displacement. This permits the basic unit of escapement to be varied, and allows the number of units per escapement cycle to vary in proportion to character width. In this paper the authors discuss the machine requirements that led to this approach and describe the elements that have evolved. The analysis used to evaluate the design (and modify it to some extent) is also recorded, in a separate section.